1. Field of the Invention
The present invention relates to an image card an image formed on a card base that is made of paper or plastic by engraving, image engraving device and image engraving method. In particular, the present invention relates to image card an image watermark-engraved by changing the thickness of the card according to a scraping-out amount and a hidden image laid on the watermark-engraved image and engraved by means of making angles of striped grooves within the card plane different, the engraved device and the engraved method capable of forming the hidden image.
2. Description of Related Art
There is an image engraving device which performs engraving on a thin plate made of paper or plastic by sweeping the plate with the engraving needle through relative movement between an engraving needle and a plate (an image card) (see Patent Literature 1). In the image engraving device, a plate to be engraved attached to a base stand (supporting stand) is reciprocated in a horizontal direction, and the engraving needle lowered in a vertical direction is brought into contact with the plate to be engraved to engrave a surface thereof so that watermark engraving is performed while changing a thickness direction of the plate to be engraved.
The plate to be engraved (image card) which has been watermark-engraved in this manner is used as a person authentication card or a security sheet for an important document (see Patent Literatures 2 and 3). For the image cards used as documents for security, there is a technical demand for showing validities of the image cards by embedding hidden images in the image cards, but there is not been any method for forming a hidden image by the engraving so far.
As the conventional art, there is a method for distinguishing an original printed matter from a copy thereof from each other by printing parallel line patterns, which are striped patterns, arranged at different angles on the printed matter (see Patent Literature 4), but there is neither a method for embedding a hidden image using a cubic shape obtained from an engraved matter nor a method for forming a hidden image and a watermark-engraving within the same plane in combination.
In view of these circumstances, it is possible to embed a hidden image by forming striped grooves with different angles, but undulations appear on wall faces of stripes according to angles of striped grooves in such a conventional image engraving device as shown in Patent Literature 1. Differences due to the stripe angles appear due to the differences of the angles of the striped grooves when the hidden image is observed on plane from above, which results in such a problem that the image does not configure a hidden image.    Patent Literature 1: Japanese Patent Application Laid-Open Publication No. 05-024394    Patent Literature 2: Japanese Unexamined Patent Application Publication No. 2007-118395    Patent Literature 3: Japanese Unexamined Patent Application No. 2007-130855    Patent Literature 4: Japanese Patent Application Laid-Open Publication No. 10-297077